1. Field of the Invention
This invention relates to methods and systems for cleansing semiconductor wafers and other items requiring extremely high levels of cleanliness, while minimizing damage to the wafer or object being cleaned.
2. Description of the Related Art
Systems employing megasonic or ultrasonic cleaning processes have been widely used to remove particles and defects from objects such as silicon wafers used in the semiconductor industry. The wafers are sometimes cleaned, for example, in a liquid or fluid into which megasonic energy is propagated. These megasonic cleaning systems safely and effectively remove particles from objects, where a system typically includes a signal generator, a piezoelectric transducer, and a transmitter, among other components. The transducer is electrically excited by a signal that causes it to vibrate, and the transmitter transmits the resulting vibration into the cleaning liquid in a processing tank. For an object such as a silicon wafer, the agitation of the cleaning liquid produced by the megasonic energy loosens particles and contaminants on the semiconductor wafers. Such contaminants are thus vibrated away from the surfaces of the wafer.
While the size of silicon chips has increased, the width of a circuit line (the line width) on the chips has become smaller in order to fit more devices on each chip. As a result, the critical particles too small to be effectively removed by older cleaning systems should be removed, but without wafer structure damage: these small particles and defects, on the order of about 0.16 μm or below, should be removed to ensure proper circuit function. At the same time, the removal process should not damage the fine structure of the chip.
A megasonic cleaning system typically creates a megasonic field, where the field is applied to an object in a cleaning fluid, such as, for example, a detergent liquid or hydrofluoric acid. The megasonic field causes bubbles to appear, pulsatingly vibrate, and collapse in the cleaning fluid. This process of bubble formation and collapse in a megasonically agitated liquid—cavitation—is the main contribution factor for effective particle removal from objects.
Cavitation is a physical phenomenon. In a liquid or other fluid energized by an acoustic field, bubbles are generated when the amplitude of negative pressure of sound waves exceeds the threshold pressure for cavitation of the liquid. Generally, the cavitation threshold is determined by the time interval of negative pressure cycles in the sound waves as they move through the liquid, along with other factors including but not limited to liquid gas content, temperature, viscosity, and liquid surface tension. Bubbles can contain vacuum, gas, liquid vapor, or a mixture thereof. The bubbles continue to pulsate and grow, and fresh gas or water vapor will continue to diffuse into the bubbles, in a process called microstreaming. Generally, negative acoustic pressure causes the bubbles to grow, and positive acoustic pressure limits the size of bubbles or provokes collapse.
Once the surface tension of a bubble is insufficient to withstand the positive pressure cycles caused by the sound waves of the applied acoustic field, the bubble collapses. The bubble collapse typically generates concentrated pressure, high temperatures, and shock waves in the cleaning liquid. The speed of bubble collapse is typically more than 300 m/sec., and high temperatures in the liquid often occur within the order of a nanosecond. As with the cavitation threshold, factors including gas content, temperature, viscosity, and liquid surface tension between the liquid and the bubbles typically influence the bubble size and density in the cleaning liquid or other fluid.
Cavitation and microstreaming, while important to wafer cleaning, also substantially increase the risk of damage to the fine structures on objects such as silicon wafers, including, for example, fine patterns on the wafers or thin films covering the wafers. Large bubbles often interact with the object to be cleaned resulting in substantial damage rather, than cleaning, where the damage often results from the violent pressure and shock waves from cavitation bubble collapse near the object. From a cleaning efficiency point of view, although a high density micro bubble field is needed to clean an object in a megasonic cleaning processes, that field must not be so strong as to damage fine structures and films on the wafer or object to be cleaned.
One solution to this problem is an increase in megasonic frequency applied to the cleaning liquid. The increase in frequency results in a shorter sonic wavelength, smaller negative sound pressure cycles in sound waves, and thus formation of smaller, less damaging bubbles. Another solution is a decrease in megasonic power. However, both of these solutions have a fundamental flaw when applied alone: although the average cavitation intensity (and hence wafer damage) is decreased in the local liquid region close to the wafer, the local bubble density decreases as well. The decrease in local bubble density hinders the cleaning effectiveness of the megasonic process. Thus, while bubble size is advantageously buffered, bubble quantity is buffered as a side effect, resulting in less effective cleaning.
While many investigations have been made into the control of various megasonic process parameters, such as, for example, changes in train time, degas time, burst time, and quiet time of sound waves, it is the use of continuous sound waves that generates the highest cleaning efficiency. So while changes in these various wave times typically modify cleaning process parameters, they cannot optimize the cavitation cleaning process: only continuous sound waves have the lowest cavitation threshold for bubble production at a selected frequency. For example, increasing quiet time or degas time for a megasonic field can decrease average cavitation density to avoid possible damage on the wafer or object, but this process decreases the efficiency of cleaning and decreases the usable wafer yield.
A need remains for a simple and practical method and device for controlled buffering of cavitation processes in an acoustic field, ultrasonic or megasonic, where enough cavitation density is generated to clean objects well while bubble size is controlled to avoid damage to objects.